Personal Alert Safety System (PASS) devices date back to the 1980's (i.e. NFPA 1982 was developed from the Technical Committee on Protective Equipment at a meeting in 1980), and PASS-type products from many vendors have been introduced over the intervening years.
PASS devices are usually worn by first responders, firemen for example, to provide a level of personal protection for such individuals in very dangerous circumstances. They usually emit an audio alarm if the wearer falls, ceases moving or the like. The intent is to identify acoustically the location of an individual that is in trouble and needs assistance. They can also emit pre-alarm and informational signals.
One common requirement for PASS devices is environmental robustness, including operation after 2 hour immersion in water (NFPA 1982-1998 Edition, “Standard on Personal Alert Safety Systems (PASS)”, Section 6-4), and high temperature operation up to +203 deg. F. after a 15 minute exposure (Section 6-12.11). One method of implementing an audio sounder for this harsh environment is to use a piezo-diaphram module.
Exemplary devices that are of a type used by first responders include:                DSX-II (2004)        MSA AirPack Integrated (1996)        MSA FireFly II (1993)        SurPass 88 (1988)        LifeGard II (1985)        PAL5 (1982)        
Market research indicates (circa-2004) that there have been about 15 PASS device vendors in the USA, and several more internationally, with a total of over 50 PASS device products. Observing that half of the test device vendors are no longer producing PASS devices, it is estimated that over 100 different PASS device products have been placed in service since 1982.
It has been recognized that PASS device audio output alarm signals are not always heard by other first responders at an emergency scene. This of course can be due to noise at the scene from a variety of sources as well as chaotic conditions often present in emergencies.
Adding to the challenge of successful PASS device detection are the acoustic conditions present at an emergency scene. The PASS device audio alarm signal sound pressure level (SPL) at 1 m. is defined to be 100 dBA for pre-alarm signals and 95 dBA for alarm signals (NFPA 1982-1982 Edition, Section 5-1.1 and 5-1.2). With fireground SPL exceeding 105 dBA, the PASS device pre-alarm signal is at −5 dB relative to the ambient SPL (i.e. caused by the fire).
Further, civilian testing indicates that the PASS device alarm signal SPL may be attenuated by 20 dB or more when the PASS device is under the body of the wearer (i.e. who is laying on the ground). In this situation, the PASS device audio alarm SPL is diminished to 75 dbA (pre-alarm diminished to 80 dB), some 30 dB (25 dB for pre-alarm) below the ambient fireground SPL. Effectively, the detection mechanism must be sensitive enough to identify the PASS device audio alarm signal having a −30 dB Signal-to-Noise Ratio (SNR) at a 1 meter distance between the PASS device and sensor.
Assuming a 9 foot ceiling (about 3 meters), and the SPL falls with the inverse square of the distance, an un-attenuated (by a body) PASS device audio alarm signal directly under a ceiling-mounted sensor would present a maximum SPL of (95 dbA−9.5 dB)=83.5 dBA. Applying body-caused attenuation (20 dB), the SPL at the sensor would be just 63.5 dBA, or nearly 40 dB below the fireground ambient SPL of 105 dbA.
Based on studies conducted of firefighter fatalities (“Firefighter Fatalities in the United States in 2003”, United States Department of Homeland Security, Federal Emergency Management Agency, U.S. Fire Administration, Aug. 2004), fatalities involving PASS device audio alarm signals occurred away from the flamefront, hence the assumed maximum SPL in the building region where detection occurs would be somewhat less than 105 dBA, and the detection SNR would therefore be greater than −40 dB.
Finally, the detection mechanism must function properly over a range of signal repetition and frequency patterns (NFPA 1982-1998 Edition, Appendix A5-2.1), while rejecting other signals having similar component frequencies and repetition rates (i.e. human speech, music, equipment noise, water and fire sounds, etc.).
There continues to be a need for systems and methods which can automatically determine the existence and location of audible outputs from PASS-type devices. Preferably such systems and methods could be integrated with new and into existing building or regional monitoring systems without requiring extensive redesign or additional hardware. It would also be desirable to be able to provide audible and/or visual indicators at monitoring system control panels so that those directing the response to the emergency will immediately be informed that one or more individuals at the scene need immediate assistance.